U.S. patent number 5,162,038 [Application Number 07/827,501] was granted by the patent office on 1992-11-10 for infant warmer open bed.
This patent grant is currently assigned to Hill-Rom Company. Invention is credited to John B. Wilker.
United States Patent |
5,162,038 |
Wilker |
November 10, 1992 |
Infant warmer open bed
Abstract
An infant warmer is provided with a heating element for
directing radiant heat onto an infant support surface and a
temperature sensing probe attachable to the skin of an infant. A
heater control which, in an automatic mode, supplies a control
signal to a heater power control which includes a component
responsive to the difference between the actual temperature and a
set temperature and a component which is added or subtracted in
incremental amounts depending on whether the actual temperature is
above or below the set point. The control also operates to reduce
the power to the heater when the total energy supplied to the
heater exceeds a maximum amount over a predetermined time interval.
In manual mode, the power level is reduced when a preset
temperature is reached or if the temperature is not reached after a
predetermined time has elapsed. In addition, the heater power is
interrupted whenever the control signal or the current to the
heater exist independent of the other. The features operate to
provide highly responsive temperature regulation and enhanced
safety.
Inventors: |
Wilker; John B. (Fairfield,
OH) |
Assignee: |
Hill-Rom Company (Batesville,
IN)
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Family
ID: |
27034082 |
Appl.
No.: |
07/827,501 |
Filed: |
January 27, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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444837 |
Dec 4, 1989 |
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Current U.S.
Class: |
600/22; 119/306;
119/319; 219/494; 219/497; 236/3; 237/3; 392/418 |
Current CPC
Class: |
A61G
11/00 (20130101); G05D 23/24 (20130101); A61G
2203/46 (20130101) |
Current International
Class: |
A61G
11/00 (20060101); G05D 23/20 (20060101); G05D
23/24 (20060101); A61B 011/00 (); H05B
001/02 () |
Field of
Search: |
;128/30,28,205.26
;600/21,22 ;236/2-4 ;119/35,39,37 ;237/2A,3,14 ;219/497,501 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Swyer, P. R., "Thermoregulation In The Newborn", The Thermal
Environment of the Newborn, pp. 773-790. .
Agate, F. and Silverman, W., "The Control of Body Temperature In
The Small Newborn Infant by Low-Energy Infrared Radiation",
pediatrics (May 1963), pp. 725-733. .
Robinson, R. O., and Jones, R., "Advantages and Disadvantages of
Overhead Radiant Warmers", Proceedings, Royal Society of Medicine
(Mar. 1977), vol. 70, pp. 209-211. .
Radiant Warmer, Narco Air-Shields Co., Infant Care Systems, Detail
Aid 7802 (Apr. 1980). .
Wheldon, A. and Rutter, N., "The Heat Balance of Small Babies
Nursed in Incubators and Radiant Warmers", Early Human Development,
Elsevier Biomedical Press, (1982), vol. 6, pp. 131-143. .
Baumgart, S., "Partitioning of Heat Losses and Gains in Premature
Newborn Infants Under Radiant Warmers", Pediatrics (Jan. 1985),
vol. 75, No. 1, pp. 89-99. .
Radiant Warmer, Ohmeda Co., BOC Health Care, Ohio Infant Warmer
Systems Tech. Information (Nov. 1985), Form No. DO16. .
Radiation Warmer, Atom Medical Corp., (Dec. 1985), V-3200N. .
Radiant Warmer, Hill-Rom Co., Stabilet 2000 Product Specification
(Nov. 1989), pp. 1-6. .
Radiant Warmer, Hill-Rom Co., Stabilet CC 1500 Product
Specification (Dec. 1989), pp. 1-6..
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Primary Examiner: Cohen; Lee S.
Assistant Examiner: Pontius; Kevin
Attorney, Agent or Firm: Wood, Herron & Evans
Parent Case Text
This application is a continuation of application Ser. No.
07/444,837, filed Dec. 4, 198
Claims
Having described the invention, what is claimed is:
1. An infant warmer bed comprising:
a base;
an infant support surface mounted on said base;
a heating circuit connectable to a source of electrical energy to
energize a heating element connected therein;
a heating element supported on said base and connected in said
heating circuit, said element being so positioned and operable to
generate heat onto said surface when electrically energized;
means connected in said heating circuit for varying the electrical
energy supplied to said element in response to a control
signal;
a control circuit having an output connected to said energy varying
means for delivering said control signal thereto, said control
circuit having an input for receiving a temperature signal; and
a temperature sensing probe positionable at said support surface
and attachable to the skin of an infant supported on said surface,
said probe having an output connected to the input of said control
circuit and operable to generate a temperature signal responsive to
the temperature sensed at said probe; and
said control circuit including
means for generating an intermediate signal in response to the
difference between the temperature signal and a predetermined
temperature setting; and
means for generating an offset signal and for algebraically adding
said offset signal and said intermediate signal so as to produce
said control signal responsive to the algebraic sum of said offset
signal and the difference between said temperature signal and said
temperature setting; and
said offset signal generating means including means responsive to
the sign of the difference between said temperature setting and
said temperature signal for periodically and cumulatively
incrementing said offset signal a predetermined amount, up or down
in accordance with said sign of said difference.
2. The infant warmer of claim 1 wherein said offset signal
incrementing means is operable to increase said offset signal in
response to said temperature signal being less than said
temperature setting and to decrease said offset signal when said
temperature signal is greater than said temperature setting.
3. The infant warmer of claim 1 wherein said control circuit
includes:
means for generating said control signal at a predetermined level,
in response to a manually initiated command signal, while said
temperature signal remains less than said predetermined temperature
setting up to a predetermined maximum amount of time after the
heating element is energized, and for thereafter generating said
control signal in accordance with said intermediate signal.
4. The infant warmer of claim 1 wherein said control circuit
further comprises:
means for generating a monitoring signal in accordance with the
energizing of said heating element;
means for comparing said monitoring signal with said control signal
and for generating an alarm signal whenever said either monitoring
signal or said control signal exists exclusive of the other;
and
means for disabling the energizing of said element in response to
said alarm signal.
5. An infant warmer bed comprising:
a base;
an infant support surface mounted on said base;
a heating circuit connectable to a source of electrical energy to
energize a heating element connected therein;
a heating element supported on said base and connected in said
heating circuit, said element being so positioned and operable to
generate heat onto said surface when electrically energized;
means connected in said heating circuit for varying the electrical
energy supplied to said element in response to a control
signal;
a control circuit having an output connected to said energy varying
means for delivering said control signal thereto, said control
circuit having an input for receiving a temperature signal; and
a temperature sensing probe positionable at said support surface
and attachable to the skin of an infant supported on said surface,
said probe having an output connected to the input of said control
circuit and operable to generate a temperature signal responsive to
the temperature sensed at said probe;
said control circuit including:
means for generating an intermediate signal in response to the
difference between said temperature signal and a predetermined
temperature setting; and
means for generating an offset signal and for combining said offset
signal with said intermediate signal to produce said control signal
responsive to said offset signal and to the difference between said
temperature signal and said temperature setting; and
said offset signal generating means including means responsive to
the relation of said temperature setting to said temperature signal
for intermittently and incrementally modifying said offset signal;
and
wherein said control circuit further comprises:
means for generating an energy level signal responsive to the
amount of electrical energy supplied to said element during an
interval of time, and for altering said control signal so as to
reduce the electrical energy to said element in response to said
energy level signal when the amount of energy supplied to said
element during said interval exceeds a predetermined energy
level.
6. An infant warmer bed comprising:
a base;
an infant support surface mounted on said base;
a heating circuit connectable to a source of electrical energy to
energize a heating element connected therein;
heating element mounted on said base connected in said heating
circuit, said element being so positioned and operable to generate
heat onto said surface when electrically energized;
means connected in said heating circuit for varying the electrical
energy to said element in response to a control signal;
a control circuit having an output connected to said energy varying
means for supplying said control signal thereto, said control
circuit having an input for receiving a temperature signal; and
a temperature sensing probe positionable at said support surface
and attachable to the skin of an infant supported on said surface,
said probe having an output connected to the input of said control
circuit and operable to generate a temperature signal responsive to
the temperature sensed at said probe; and
said control circuit including
means for generating said control signal in response to said
temperature signal so as to maintain the temperature at said probe
at a predetermined temperature setting; and for generating an
energy
means for generating an energy level signal responsive to the
amount of electrical energy supplied to said element during an
interval of time, and for altering said control signal so as to
reduce the electrical energy to said element in response to said
energy level signal when the amount of energy supplied to said
element during such interval exceeds a predetermined energy
level.
7. The infant warmer of claim 6 wherein said control circuit
further comprises means for resetting said energy level signal at
the end of said interval of time, said energy level signal
generating means being operative to generate said energy level
signal responsive to the amount of electrical energy supplied to
said element during a second interval of time subsequent to the
resetting by said resetting means.
8. The infant warmer of claim 6 wherein said control circuit
includes:
means for generating said control signal at a predetermined level,
in response to a manually initiated command signal, while said
temperature signal remains less than said predetermined temperature
setting up to a predetermined maximum amount of time after the
heating element is energized, and for thereafter generating said
control signal in accordance with said intermediate signal.
9. The infant warmer of claim 6 wherein said control circuit
further comprises:
means for generating a monitoring signal in accordance with the
energizing of said heating element;
means for comparing said monitoring signal with said control signal
and for generating an alarm signal whenever said either monitoring
signal or said control signal exists exclusive of the other;
and
means for disabling the energizing of said element in response to
said alarm signal.
10. An infant warmer bed comprising:
a base;
an infant support surface mounted on said base;
a power circuit connectable to a source of electrical energy;
a heating element mounted on said base connected in said power
circuit to be energized thereby, said element being so positioned
and operable to generate heat onto said surface when electrically
energized;
means connected in said power circuit for varying the electrical
energy to said element in response to a control signal;
a control circuit having an output connected to said energy varying
means for supplying said control signal thereto, said control
circuit having an input for receiving a temperature signal;
a temperature sensing probe positionable at said support surface
and attachable to the skin of an infant supported on said surface,
said probe having an output connected to the input of said control
circuit and operable to generate a temperature signal responsive to
the temperature sensed at said probe; and
said control circuit including:
means for generating an intermediate signal in response to the
difference between said temperature signal and a predetermined
threshold signal, and
manual startup means for generating said control signal at a
predetermined level, in response to a manually initiated command
signal, said manual startup means being operative only while said
temperature signal remains less than said threshold signal and for
up to a predetermined maximum amount of time after the heating
element is energized, and
automatic temperature control means operative after said
temperature signal equals said threshold signal and after said
predetermined maximum amount of time after the heating element is
energized, and inoperative when said manual startup means is
operative, for generating said control signal in accordance with
said intermediate signal.
11. The infant warmer of claim 10 wherein the predetermined level
is a constant level.
12. The infant warmer of claim 10 wherein said control circuit
further comprises:
means for generating a monitoring signal in accordance with the
energizing of said heating element;
means for comparing said monitoring signal with said control signal
and for generating an alarm signal whenever either said monitoring
signal or said control signal, exists exclusive of the other;
and
means for disabling the energizing of said element in response to
said alarm signal.
Description
The present invention relates to infant care beds and more
particularly to infant warmer open care beds for providing
temperature regulation of infants in maternity and newborn care
facilities.
BACKGROUND OF THE INVENTION
Neonatal facilities of hospitals require, under current care
standards, the provision of infant beds which will automatically
maintain the baby's environment within acceptable temperature
limits. This necessitates the use of heating elements with controls
which will effectively regulate the heat delivered to the unit in
response to sensors in the bed which monitor the temperature in the
vicinity of the baby and, preferably, the actual temperature of the
baby's skin. It is desirable that the controls for such warmers
have, in order to reliably regulate the baby's temperature, a
stable and accurate control, and one which will also be responsive
to abnormal and potentially unsafe conditions which require an
alarm or other response.
Two types of controls have been available to regulate the
temperature of infant warmers. One is the proportional or analog
control by which power to a heater is varied as a function of the
difference between the actual sensed temperature and a temperature
setpoint. The other is the on/off type control or switching control
which alternately energizes the heater, usually at a single power
level, and switches off the heater, depending on whether the actual
temperature is either below or above a setpoint, or below a lower
minimum setpoint or above an upper maximum setpoint.
In proportional control, the temperature is usually controlled by
gradually reducing the heater output as the setpoint is approached
and/or exceeded, eventually turning off the heater when a maximum
temperature is reached. If the temperature range around the
setpoint over which the heater output is varied is wide, a very
stable temperature control can be achieved, at least where the
ambient conditions are stable. This is known as a loose
proportional control. However, the amount of heat required to
maintain the setpoint is a variable depending on external
conditions which affect the rate at which heat energy is lost from
the bed or unit. Where ambient conditions vary, loose proportional
control may equilibrate at a temperature other than the setpoint.
The equilibrium temperature which such a control may produce can,
depending on ambient conditions, equal any temperature in the range
of temperature over which the heater power is being tapered. The
conditions which affect the equilibrium temperature would include
such ambient conditions as air movement (wind chill), the
proximating of attending or visiting personnel, or the operation of
building heating and air conditioning equipment. Thus, although
proportional control is desirable for stability, it is undesirable
for accuracy.
In ON-OFF type of control, the temperature is regulated by
switching the heater to full power until the measured temperature
exceeds the setpoint by a specified amount, then switching the
heater off until the temperature drops below the setpoint by a
specified amount. This type of control continually overshoots and
undershoots the setpoint by the amount of hysteresis which is
dependent on the heater power level and the heat loss rate of the
unit. While this type of control at least brings the temperature
completely to the set point, the temperature oscillates about the
setpoint so that it is neither smooth nor stable.
Infant warmers of the prior art possess at least one of the
disadvantages of the proportional or on/off control systems
described above.
Furthermore, although most infant warmers have high and low
temperature alarms, incidents of overheating still occur with prior
art devices due to the temperature sensing probe being partially
dislodged from the skin of the infant or misapplied in such a way
that the probe senses a slightly lower temperature than the actual
skin temperature. In addition, the probe can be completely off the
infant but positioned in such a manner as to be heated by the
heating element, which is often a radiant element, to the
temperature setpoint when the temperature of the infant is actually
above or below the setpoint. All radiant warmers with servo control
are subject to this hazard. When this problem occurs, since the
temperature sensed may be within an allowable range of values, no
alarm would be activated and the heater output may be regulated at
too high or too low a temperature. Alarm systems based on
temperature rely on the skin probe to sense the correct skin
temperature of the infant. If the probe is not installed correctly,
the alarms may not respond to the proper alarm condition.
Also, many radiant infant warmers have a manual mode of operation
for use in situations where it is impossible or difficult to use
the skin probes. This may be the case in a labor-delivery room, for
example. When operating in such a manual mode, the temperature
display is normally still active and reads whatever temperature the
probe may be sensing as it rests in the warmer bed. Some warmer
beds have a slide switch to select between servo and manual modes.
This presents a possible hazard condition in that the warmer may be
in manual mode and displaying a temperature that is within the
range of an infant's skin temperature. In fact, the skin probe may
be attached to the infant while the nurse may not notice that the
switch is in manual mode. Depending on the manual heat setting, the
infant may receive too much or too little heat.
In some heater beds, the heater power is controlled by a thyristor
semiconductor device commonly known as a triac. A common failure of
thyristors is to become shorted or to turn on when they should be
off. This may result from electrical disturbances on the power
lines. When this occurs, the heater turns on and is uncontrollable.
This is of course very hazardous.
Each of the problems described above are problems currently
existing in the art. Accordingly, there exists a need in the art to
overcome the problems.
SUMMARY OF THE INVENTION
It is a primary objective of the present invention to provide for
the regulation of the temperature of an infant in an infant warmer
bed in a manner which is both accurate and stable.
It is a further objective of the present invention to provide an
infant warmer bed control which reliably protects the infant
against overheating.
It is an additional objective to adequately protect the infant in
an infant warmer bed from improper temperature regulation in the
case of use of the warmer in a manual mode.
It is a still further objective of the present invention to provide
a control which protects the infant in the case of heater control
malfunction.
According to the principles of the present invention there is
provided a temperature control having the characteristics of an
analog proportional control without the error associated with a
conventional proportional control. This is accomplished by
employing a circuit that adjusts the heater output up or down over
a limited range until the proportional control equilibrates at a
predetermined setpoint temperature. The adjustment made in this way
compensates for changes in heat loss rate due to drafts and other
such factors.
In accordance with the preferred embodiment of the invention, the
control is initialized when powered up such that the heater output
will be delivered at an intermediate value, for example 50% of
maximum power, when the measured temperature is equal to the
setpoint temperature. The actual amount of heater power, however,
may be offset from between 10 and 90% of maximum controllable power
range. Then, at widely spaced time intervals of, for example, two
minutes, the temperature offset is incremented either up or down
depending on whether the measured temperature is above or below the
setpoint.
Also in accordance with principles of the present invention, an
alarm is provided to signal excessive heating, to protect the
infant. It functions by monitoring the amount of total heat
delivered over a specified period of time. If the amount of heat
delivered is more than normally expected as, for example, may be
predetermined from statistical data for the particular type of
warmer, then the system actuates an alarm and the power is reduced
to a value predetermined to be at or below a maximum safe value,
for example, at half power.
Further, in accordance with principles of the present invention, a
control for an infant warmer bed is provided with an automatic
reset to a manual mode when the system is first powered up or when
a prewarm option is selected by, for example, the pressing of a
button. The control, however, immediately switches to a servo or
automatic temperature control mode when the skin probe senses a
temperature the set-point temperature, preferably a temperature of
31.degree. C. In accordance with the preferred embodiment of the
invention, the temperature display is turned off if the unit is in
manual. Then, when the system senses warmth on the temperature
probe, the system latches into servo mode, turns on the temperature
display and activates all safety alarms. Thus, the likelihood of
having the system in manual mode and thinking it is in servo mode
is substantially reduced.
Still, in accordance with other principles of the present invention
a heater current signal proportional to the actual heater current
is compared with the heater control signal from the proportional
controller. If, at any time the two signals do not agree, a system
fault alarm is activated. Preferably, the system fault alarm
removes AC power from the heater by means of an electromagnetic
relay and sounds an audible alarm and lights all fault
indicators.
The present invention provides an infant warmer bed with stable and
accurate temperature regulation, and with protection for the infant
against overheating, inadvertent manual mode operation and power
circuit failure.
These and other objectives and advantages of the present invention
will be more readily apparent from the following detailed
description of the drawings in which:
FIG. 1 is a perspective view of one preferred embodiment of an
infant warmer bed in accordance with the principles of the present
invention.
FIG. 2 is a view of the control panel of the embodiment of FIG.
1.
FIG. 3 is a schematic block diagram of a control circuit of the
embodiment of FIG. 1.
FIG. 4 is a circuit block diagram of portions of the circuit of
FIG. 3.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to FIG. 1, an infant warmer bed 10 according to a
preferred embodiment of the present invention is illustrated. The
unit 10 includes an upstanding base 11 which has supported thereon
a horizontal procedure table or bed 12. Supported at the top of the
base or frame 11 is a warmer and lighting module 14 which includes
a radiant heating element 15 directed downwardly onto the surface
of the bed 12 so as to warm the bed 12 when energized. The bed 12
is enclosed on its sides by upstanding walls 16 with an open top
17. Above the bed 12 on the frame 11 is a panel 18. Connected
through a cord to the panel is skin temperature probe 20 attachable
to the infant's skin for sensing the baby's skin temperature The
panel 18 is illustrated in detail in FIG. 2.
Referring to FIG. 2, the control panel 18 according to the
preferred embodiment is illustrated. On the control panel 18 are
mounted controls which include a power on-off push button switch
21, a lighting on-off push button switch 22, an Apgar timer on-off
push button switch 23, a temperature setting control 24, a servo
mode heater power indicator 25, an infant temperature display 26, a
heater power control manual mode silence push button switch 29, and
alarm indicators including an overheating indicator 30, "hi/lo"
temperature indicator 31, over 39.degree. C. indicator 32, and
probe fault indicator 33. The controls and the indicators of the
control panel 18 are elements of the control circuit illustrated in
the circuit block diagram of FIG. 3.
FIG. 3 illustrates the controller operated from the control panel
18 to control the heater element 15. The control includes a control
circuit 34 connected between the controls on the panel 18 and a
heater power circuit 35 which energizes the heater element 15. The
heater element 15 is connected into the circuit 35 across the leads
of a supply cord 36 connectable to a source of AC power, typically
120 VAC 60 Hz. The heater element 15 is connected in the circuit 35
in series with a heater power control triac 37. The triac 37 has a
control input 38 connected to the output of a triac driver circuit
39. The triac driver circuit 39 has an input connected to the power
control output 40 of the control circuit 34. A normally closed
cut-off relay 41 is also connected in series with the heater 15 and
the triac 37 in the circuit 35. The relay 41 has an actuator 42
which is coupled to operate in conjunction with the system fault
alarm 43.
A Hall-effect current sensor 44 is provided for generating a signal
at a monitoring input 45 of the control circuit 34. The detector 44
has a sensor input 46 coupled to a current responsive indicator 47
connected in series with the heater circuit 35.
A reset circuit 48 is provided having an input 49 coupled to
circuit 35 to provide a power-up reset momentary signal at a reset
input 50 of the circuit 34. The reset circuit 48 has a second input
from the alarm silence pushbutton 29. Operation of the alarm
silence pushbutton 29 also causes a reset signal to be generated on
a reset line 50 to the circuit 34.
The control circuit 34 includes four subcircuits which include a
self-adjusting proportional servo-control circuit 57, a heater
power servo/ manual/cutback selector circuit 59, an automatic mode
control circuit 60 and a "hi/lo", 39.degree. C., probe, overheat
system fault alarm circuit 61.
The skin temperature probe 20 is connected to the input of a
temperature measuring circuit 51 which has an output 52 on which a
signal proportional to the measured skin temperature is delivered
to a temperature signal inputs of the proportional control circuit
34 and the alarm circuit 61 of the control circuit 34. The
temperature setting control 24 is connected to a temperature
setpoint input 53 of the proportional control 57 and alarm circuit
61 of the control circuit 34. The manual mode heater power setting
control 27 is connected to a manual power level input 56 ,of the
control circuit 34.
The self-adjusting proportional control 57 has a pair of inputs
connected one to the temperature signal output 52 of the
temperature measuring circuit 51 and the other to a setpoint signal
line 53 of the servo temperature setpoint control 24, and an output
for delivering an analog signal on a line 58 to an input of the
heater power selector circuit 59.
The heater power selector 59 is a dual analog switch circuit which
selects one of three signals from either of three inputs connected
respectively to the proportional control circuit 58, a manual
control line 56 from the manual control 27, or a preset default
level signal line 30 in response to signals from the output 62 of
the automatic mode control circuit 60 or from an output 54 the
alarm circuit 61. The output signal on output line 40 of the power
selector circuit 59 is delivered to the input of the triac driver
circuit 39. The triac driver circuit 39 is an optical coupled triac
which operates to deliver a gate signal on the gate input 38 of the
heater power control triac 37 to cause it to control the power in
the power circuit 35 in accordance with the analog signal output 40
of the control circuit 34.
The automatic mode control circuit 60 has three inputs, one of
which is connected to the temperature signal input 52 of the
circuit 34, another of which is connected to the reset signal input
50 of the circuit 34, and a digital output signal line 62 connected
to the alarm circuit 61 and the heater power selector circuit
59.
The control circuit 34 has as its fourth subcircuit, an alarm
circuit which has seven inputs and six outputs.
The inputs to the alarm circuit 61 are reset input from reset line
50, the servo/manual mode signal input from the output 62, a
"31.degree. C." signal input 108 from the mode control circuit 60
of control circuit 34, a temperature signal from line 52, a
measured temperature-setpoint input 68 from the servo-control
circuit 57, the heater drive signal from output line 40 of circuit
34, and an input from the heater current signal input 45 from the
hall effect pick-up circuit 44. The outputs from the alarm circuit
61 are a hi/lo temperature alarm signal output 63, a 39.degree. C.
alarm signal output 64, a probe fault signal output 65, an
overheating alarm signal output 66, a preset power signal output 54
to the heater power selector circuit 59 and a system fail signal
output 55 to the system fail indicator 43 and cutoff relay 41. The
output signals 63,64,65,66 drive respective alarm indicators
31,32,33,30 (not shown) on the panel 20.
The reset circuit 48 has two inputs, one is the alarm silence,
pushbutton 29, the other is a sensor input 49 sensing the presence
of line power 49. The circuit 48 has one output 50 connected to the
control circuit 34. A power loss alarm circuit 67 is provided for
sensing the loss of AC power circuit. The circuit 67 has an input
connected to sensor line 49 for sensing the presence of AC line
power, and has an audible signaling device contained therein for
producing an audible alarm output.
The details of the subcircuits 57, 59, 60 and 61 and of the related
circuitry are illustrated in greater detail in the block diagram of
FIG. 4.
Referring to FIG. 4, the self adjusting proportional control
circuit 57 is illustrated. It includes a differential amplifier 70
having a pair of inputs connected one each to the temperature
signal input line 52 and the temperature setpoint signal line 53.
The circuit 57 also has an output signal line 71 connected to a
summing network 72 and to an input of the alarm subcircuit 61. The
circuit 57 further includes an analog comparator circuit 73 which
is provided also with a pair of inputs connected, one each to the
temperature signal 52 and the setpoint signal 53, and having an
output 74 connected to an up/down control input of a 4 bit binary
up/down counter 75. The comparator 73 produces a digital signal
output 74 indicative of which of the two analog signals connected
to the inputs, 52 & 53 is greater in voltage.
The counter 75 has a clock input 76 connected to the output of a
long time interval timer circuit 77. This causes the counter 75 to
step up or down in accordance with the state of the up/down input
74 at a slow rate, for example every two minutes. The counter also
has a preset input 78 which sets the counter to an intermediate
value, for example "8" upon power-up of the unit 10. The counter 75
has a parallel 4 line digital output 79 connected to a digital to
analog converter circuit 80 which has an analog output connected to
the second input of the summing network 72. The summing network
adds the two analog signals from the differential amplifier 71 and
the D/A converter 80 to produce the servo control signal 58.
The servo heater control output 58 of the summing network 72 is
connected to one of three analog inputs of the heater power
selector circuit 59. The second of three analog inputs 56 is
connected to the output 56 of the manual control 27. The third
analog input 30 is a preset level formed by a voltage divider which
is the level set during an alarm condition.
The heater power selector subcircuit 59 is controlled by a digital
signal 62 from the automatic mode control circuit 60 and a second
digital signal 54 from the alarm circuit 61. The circuit 59
includes a pair of analog switches 1-2 and 113. The function of the
switches 112 and 113 is to connect one of three analog inputs
(servo, manual or preset) to the output 85. The output 85 is
connected to the input of a voltage comparator 87 which has a
second input 84 connected to a sawtooth wave generator 83. The
voltage comparator 87 produces a pulse width modulated square wave
output 40 whose ratio of "on" time to "off" time instantaneously
varies with respect to the difference between the actual
temperature and the setpoint temperature and varies in a slow
stepwise manner to the difference between the setpoint and the
equilibrium point temperature.
The modulated square wave output 40 of the control circuit 34 is
connected via the opto-coupler triac driver circuit 39 to the
heater power control triac 37. The analog output 85 is also
connected to a bargraph driver circuit 86. The bargraph driver
circuit illuminates a series of rectangular LED bars to display the
power level. The bargraph driver is connected to two bargraphs, one
for servo mode 25 and the other for manual mode 27. Only one of the
two bargraphs are on at a given time as determined by the mode
control signal 62.
The automatic mode control subcircuit 60 has an input from the
temperature signal 52 connected to a comparator circuit 107
designed to produce an output signal 108 when the measured
temperature is greater than 31C. The comparator output 108 is
connected to the set input of a latch circuit 110 and to the input
of a one-minute timer 121 of the alarm circuit 61. The latch
circuit output 62 is connected to the heater power selector circuit
to switch the control from manual to servo operation and the alarm
circuit to enable the hi/lo alarm 31 and the probe dislodged timer.
The hi/lo temperature and dislodged probe alarms 31 and 32 are
active only in servo mode. The servo mode latch circuit 110 is
reset to manual mode by a reset signal 50 which is active whenever
the unit is powered up or the alarm silence button is pressed.
The alarm circuit 61 consists of five separate alarms: a hi/lo
temperature alarm 31, a 39.degree. C. temperature alarm circuit 32,
a probe alarm 33 (open or dislodged), an overheating alarm 30, and
a system fault alarm 43. The alarm signals from the first four
alarms 30-33 are connected to an OR gate 122 whose output is a
power cutback signal generated on signal line 54 which causes the
heater power selector circuit 59 to switch to the preset level
regardless of the state of the automatic mode circuit. The preset
level is set to approximately 25% of full power.
The hi/lo temperature alarm circuit consists of two voltage
comparators 97 & 98 which compare the output signal on, line 71
of the differential amplifier 70. This signal corresponds to the
difference between the measured temperature and the temperature
control setpoint to preset voltage levels equivalent to
+0.5.degree. C. and -0.5.degree. C. temperature differential. The
outputs of the two comparators are connected to an OR gate 81 so
that either output will cause an alarm signal to be produced to one
of two inputs of an AND gate 82 if the measured temperature differs
from the control setpoint temperature by +/-0.5.degree. C. Another
input of the AND gate 82 is connected to the servo/manual signal 62
such that the hi/lo alarm 31 is enabled only in servo mode. The
output 63 of the AND gate 82 is connected to the hi/lo alarm
indicator 31 and to the enable input of a two minute timer 119. The
output of the timer 119 is connected to an input of an OR gate 122.
If the hi/lo alarm signal on line 63 is on for two minutes or more,
the timer output 119 is activated which causes the OR gate output
54 to activate which sets the heater power circuit 35 to the preset
power level.
The 39.degree. C. alarm consists of a voltage comparator circuit
116 with the measured temperature signal 52 as one input and a
voltage divider set to a voltage level equivalent to a 39.degree.
measured level as the second input. The output 64 of the comparator
116 is connected to the 39.degree. C. alarm indicator 32 and to the
OR gate 122. If the measured temperature exceeds 39.degree. C., the
39.degree. C. alarm indicator 32 is activated and the heater power
is set to the reduced power mode.
The probe alarm 33 consists of a one minute timer 121 which is
enabled by the servo mode signal on line 62 and the inverse of
31.degree. C. temperature signal on line 108. If the unit is in
servo mode and the probe temperature drops below 31.degree. C. for
1 minute or longer, a signal from the output of the timer 121 is
connected through an OR gate 120 to the probe fault output 65. The
output 65 is thereby activated and the heater power is set to the
preset level of approximately 25%. The one minute delay allows the
probe location to be changed without activating the alarm. If the
probe becomes open circuited, for example due to a broken wire,
then the probe fault output 65 is immediately activated by the open
probe comparator 117, the output of which is connected through the
OR gate 120 to the output 65, and to an input of the OR gate 122 by
which the heater power is reduced to the preset level. The timer is
reset by the alarm silence & power-up reset signal 50.
The overheating alarm includes a square wave circuit 88 having an
input connected to the heater current signal 45 from the
Hall-Effect current pickup circuit 44. The square wave circuit 88
functions to amplify and hardlimit the AC signal from the
Hall-Effect device 44, to produce a rectangular wave signal
consisting of bursts of 60 Hz pulses for a duration proportional to
the power delivered by circuit 35 to the heater 15. These pulses
are connected to the clock input of a 24 bit binary counter 90. A
16 bit parallel digital output 91 of the counter 90 is connected to
a programmable decoder 92 which generates an overheating alarm
output signal 66 when the count of the counter 90 exceeds a
predetermined number indicative of excessive total power to the
heater during a predetermined time interval. The time interval is
determined by a timer 93 which is preferably a 15 minute timer,
connected through an OR gate 113 to the reset input 94 of the
counter 90. The other input of OR gate 113 is connected to the
reset timer 50. If the counter 90 fails to reach the predetermined
count during the duration of the time period, the counter is reset
and the cycle repeated. The overheating alarm output 66 is
connected to the overheating alarm indicator 30 and to the OR gate
112 which causes the heater power to be set to the preset level if
the alarm condition is present.
The system fail alarm circuit monitors the power control circuitry
35 for the heater 15. The signal on line 89, consisting of bursts
of 60 Hz square wave pulses, is connected to a 20 millisecond
retriggerable multivibrator circuit 95 whose output 96 is a
rectangular waveform whose width is equal to the timer period that
the heater 15 is actually on. The multivibrator output 96 is
connected to one input of a two input exclusive-OR circuit 114,
which has its second input connected to the heater control signal
line 40 of the control circuit 34. The exclusive-OR circuit
produces an output signal on line 99 to the input of an OR gate 123
whenever the power control signal on line 40 and the multivibrator
output 96 representing heater current are not concurrent. The
output of OR gate 123 is connected to the set input of a system
fail latch circuit 100. The other input of OR gate 123 is connected
to the shorted-probe voltage comparator output 115, which compares
the signal at its input from line 52 with a voltage reference to
recognize a shorted probe condition. The system fail latch 100 has
an output 55 connected to the system fail circuit 43 which
activates the power cut-off relay 41 and illuminates all the fault
indicators 30,31,32,33. The latch circuit 100 has its reset input
118 connected to a power up reset signal 50.
AUTOMATIC SWITCHING TO SERVO MODE
In manual operation, the measured temperature by the probe 20 must
remain below 31.degree. C. During this time, the heater power is
controlled by a voltage level derived directly from the setting of
the front panel manual power level control 27. The open or shorted
probe alarms, 33, the excessive heating alarm 30, and the system
fault alarm 43 are active. If the probe temperature increases above
31.degree. C., such as would be the case if placed on an infant,
the automatic mode control switches to servo mode and latches
unless reset by the alarm silence button 29 or by powering down the
unit and turning back on. If the control is reset to manual with
the probe temperature greater than 31.degree. C., it will
immediately switch back to servo mode. The manual power bargraph
indicator 28 is illuminated only during manual operation and
indicates the percentage of full power of the heater 15.
SELF ADJUSTING PROPORTIONAL CONTROL
In servo operation, the setpoint temperature and the measured
temperature are each converted to an electrical voltage level with
equal scale factors and applied at the respective inputs 53 &
52 of the control circuit 34. These two signals are compared by the
op-amp voltage comparator 73. The output 74 of the comparator 73 is
high or low depending on which of the two voltages is larger. This
output signal 74 is communicated to the up/down control input of
the 16 count binary counter 75. The counter 75 will have been
present to count 8 when powered up and is clocked at a very slow
rate of 1 clock every 2 minutes as determined by the timer 77. The
binary counter will count up if the setpoint is above the measured
temperature, and will, count down if the setpoint is below the
measured temperature. Under steady state conditions the up/down
switching will alternate at a 2 minute rate. The counter is
inhibited from going any further if a count of 0 or 15 is reached.
The output signals from the counter 75 are connected to a
digital-to-analog converter circuit 80 which converts the count to
a voltage 81 with a level proportional to the count of the counter
75. This level is scaled such that each count produces a voltage
change equivalent to approximately 0.05.degree. C. This voltage 81
is added as an offset to the signal 71, representing the difference
between the measured temperature and setpoint voltages, by the
summing network 72 which produces the servo mode heater control
signal 58. Thus, the heater output, with the control at
equilibrium, will be adjusted slightly up or down depending on
whether the setpoint is above or below the measured temperature.
This will continue until the control comes to equilibrium precisely
at the setpoint. Once this occurs, the proportional control is
precisely compensated for the environmental conditions which
influence the rate of heat loss from the unit 10, yet will respond
immediately to measured temperature variations from the sensor
20.
PROTECTION FROM OVERHEATING OF PATIENT
The heater output is controlled by switching the heater on and off
for a number of cycles of the line current as determined by the
proportional control. The ratio of ON time to the total time
available is related to the percentage of maximum power. By
monitoring the 60 Hz heater current signal during a definitive time
interval, the average percentage of full heater power is
determined. A heater current signal is developed by passing the
current through an inductor coil 47 (FIG. 3). The magnetic field
set up by the heater current passing through the inductor 47 is
detected by a Hall-Effect device 44 which has an electrical output
45 proportional to the magnetic field set up by the heater current
in circuit 35. The heater current signal is used by the heat alarm
circuits 61 which generates both the excessive heating alarm signal
on output 66 and the heater power control monitor which controls
the system fail alarm at output 55.
The heater current signal from input 45 is converted to a square
wave and used to clock the totalizing counter-decoder 90 that will
output a signal at a predetermined count. The counter will count
the 60 Hz current signal in bursts equal in duration to the ON time
until the count equals the predetermined value or is reset. For
example, if the heater is on 50% power for 15 minutes then the
count would be:
The counter 90 is reset at preset time intervals determined by the
timer 93. If the counter 90 reaches the preset count total within
the preset time interval, then the circuit 61 activates the alarm
30 and reduces power to a safe value by switching to a preset level
of 25% for full power. If the counter 90 does not reach the preset
count within the time interval, it is reset to start a new
monitoring period. For example, if the counter is reset every 15
minutes, and the preset count to alarm is 27000, then if the power
level averaged more than 50% it would alarm.
The following are example cases:
At 100% power the alarm will go off in 7.5 min.
(100/100.times.60.times.60.times.7.5=27000)
At 60% power the alarm would go off in 12.5 min.
(60/100.times.60.times.60.times.12.5=27000)
At 40% power level the alarm would not go off. The counter 90 would
be reset to 0 at 15 minutes prior to the count reaching 27000.
(40/100.times.60 Hz.times.60 sec/min.times.15 min=21600)
Therefore, the system will alarm if the heater output exceeds a
preset amount during a predetermined time interval. The actual
values for the percentage of power and time interval are adjustable
to allow for variation in heaters,, but should approximately equal
to that percentage of heater power needed to produce an irradiance
of 20 milliwatts per square centimeter measured 4 inches above the
bed 12 for 15 minutes.
The skin temperature is converted to a voltage which is displayed
on the digital display 26 using a DVM (digital voltmeter) circuit.
This voltage also goes to the alarm board. A series of four voltage
comparators 117, 107, 116 and 115 detect the following
temperatures: (1) out-of-range low--open probe alarm 117, (2)
greater than 31.degree. C.--servo mode active 107, (3) greater than
39.degree. C. --high temp alarm 116, and (4) out-of-range
high--shorted probe alarm 115. When the unit is turned on, the
servo-manual latch 110 is placed in its manual mode by a reset
signal from reset input 50. Whenever a skin probe temperature of
31C or greater is sensed by the sensor 20, the latch 110 sets to
servo mode. If the skin probe temperature is above 31.degree. C.,
then drops below 31.degree. C., the latch 110 will stay in servo
mode until the alarm silence button 29 is pressed.
PROTECTION AGAINST LOSS OF HEATER CONTROL
The heater current monitoring is provided by passing the heater
current through the inductor coil 47 which is magnetically coupled
to a linear Hall-effect device 44. The output of the Hall-effect
device is an AC voltage proportional to the heater current. This
,voltage is converted to a square wave in the circuit 88 and is
used to trigger the 20 millisecond retriggerable one-shot
multivibrator 95. Since the period of the 60 Hz current waveform is
16.6 milliseconds, the output of the multivibrator 95 is a constant
high logic level when the heater is on and low when it is off. This
level is compared using an `exclusive OR` logic circuit with the
heater control signal from the analog output 40. If the two signals
differ, the exclusive-OR circuit triggers a latch circuit 100. The
latch 100 will also activate if the heater current is off when it
should be on. Thus, it immediately latches if the heater, fuse or
thermal switch fails open circuited. Thus, a signal is also
generated before the infant gets cold or low temperature alarm goes
off.
The present invention represents a significant improvement in the
safety of infants nursed under radiant warmers.
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